Three-dimensional (3D) printing is becoming a powerful manufacturing technology for creating sophisticated, bespoke and low-cost objects that, if possible to manufacture at all, were traditionally made by complicated procedures and facilities.1 The technology, with the capability of substantially changing the way in which materials are turned into devices, has recently been employed to develop various applications in both scientific and engineering fields such as tissue and scaffold engineering,2-7 microfluidics,8 chemical synthesis,9 composites,10,11 and electronics.12-14 Several layer-by-layer printing strategies, based on different rapid prototyping technologies such as fused deposition modeling, polymer jet deposition, granular materials binding, and stereolithography, together with a variety of material/ink systems, have been developed to meet different requirements.15 